50 



tiske Bevægelser, der med stor Regelniæssighed for- 

 plantes langs ad dens hele Længde, ialmindeliglied 

 i Retningen bagfra fortil. 



Circulationssystemet. — Hjertet danner (se Tab. 

 VI, Pig. 2, 3, 1 1 r) et overordentlig langstrakt, med 

 bestemte Mellemrnm indsnoret Ror, der strækker sig 

 ovenfor Tarmen fra Nakkesegmentet og bagtil lige 

 ind i Halens næstsidste Segment. Det er ved tine 

 jVInskeltraade fæstet til Indsiden af Ryggens Inte- 

 gument, og viser for hvert Segment et Par venose 

 Spaltaabninger, hvorigjenneni Blodet optages i Hjer- 

 tet. Den samlede Blodmasse, som paa denne Maade 

 trænger ind i Hjertet, ndstodes dels fra den forreste, 

 dels fra den bågeste Ende, og kommer derpaa ind i 

 væglose Htilrnm niellem de forskjellige Organer. 

 Skjondt saaledes et egentligt Ivarsy.stem, bestaaende 

 af Arterier og A'^ener, ganske mangler, circnlei'er dog 

 Blodet med stor Regelniæssighed omkring i Lege- 

 met, idet det folger visse bestemte Baner. Blodet 

 er, som sædvanligt, farvelost og indeholder en 

 Mængde smaa, tildels amoboide Blodlegemer, hvis 

 Gang let lader sig forfølge hos det levende Dyr 

 under Mikroskopet. Hjertets Pulsationer, hvorunder 

 Spaltaabningerne afvexlende aabnes og lukkes, er 

 særdeles livlige, saa det er meget vanskeligt at 

 tælle, hvormangp der sker i Minuttet. 



Nervesystemet. — ( "entraldelene af Nervesyste- 

 met bestaar, som hos andre Crustaceer, af et dorsalt 

 Parti, det øvre Svælgganglion eller Hjernegangliet, 

 og af en ventral Del, den saakaldte Bugganglie- 

 kjæde, begge forbundne ved en ondiring Spiserøret 

 lobende ("ommissur (se Tab. VIII, Fig. 1). 



Hjernegangliet (se Fig. 1, 4, 17), der har sin 

 Plads i den forreste Del af Hovedet, lige foran Be- 

 gyndelsen af Tarmen, er af temmelig betydelig Stør- 

 relse, og bestaar af 2, i Midten med hinanden for- 

 bundne .symetriske Halvdele. (Jventil danner det 

 flere Lappe, der delvis omfatter Basis af det enkle 

 Øie (se Fig. 4), og til hver Side fortsætter det sig 

 i den mægtigt itdviklede Synsnerve (o), etterat have 

 udsendt en tynd Nerve til Iste Par Antenner ,a'). 

 Et andet Par Nerver sees at udgaa fra Hj ernegang- 

 liet længere fortil, til hver Side af det enkle Øie. 

 De ender hver med en liden ganglios Opsvulmning 

 umiddelbart under en liden grubeformig Fordybning 

 i Hovedets forreste Integument (a), rimeligvis sva- 

 rende til de omtrent paa samme Sted lios Lininctis 

 forekommende eilierede (xruber, der ialmindelighed 

 ansees for et Slags Sandseorgan. Nerverne for 2det 

 Par Følere (a-), der hos Hannerne er l)etydelig 

 stærkere end hos Hunnerne, udspiinger fra den for- 

 reste Del af Svælgconimissurerne. Disse sidste for- 

 bindes, inden de omfatter Spiserøret, ved en tynd 

 Tværeommissur (se Fig. 1 og 4), der ligger ved Basis 

 af (Jverlæben. 



be observed, these being transmitted with great 

 regularity throughout its entire lengtli, generally 

 from behind forwards. 



Circulatory system. — The heart (see PI. VI, 

 hgs. 2, 3, lie) consists of an exceedingly elongated 

 tube, constricted at regular intervals, and extending 

 above the intestine fi'om the cervical segment back- 

 wards into the penultimate segment of the tail. It 

 is attached by tine muscle-fibres to the inside of the 

 dorsal integument, and exhibits, in each segment, 

 a pair of venous ostia, through which the blood 

 is received into the heart. The accumulated mass 

 of blood, which thus forces itself into the heart, is 

 expelled jiartly from the anterior, partly from the 

 ' posterior end, and tlien enters the several cavities 

 between the various organs. Although there is thus 

 a total absence of a true vascular system consisting 

 of arteries and veins, yet the blood circulates with 

 great regularit_y through the body, following certain 

 fixed courses. The blood is, as usual, colourless, 

 and contains a number of small, partly annebous 

 l)lood-cor])uscles, whose course may be easily traced 

 under the microscope. The pulsations of the heart, 

 dui'ing which the ostia alternately open and close, 

 are exceedingly rajiid, so that it is very difficult to 

 count the number occurring in a minute. 



Nervous system. — The central portion of the 

 nervous system consists, as in other Crustaceans, 

 of a dorsal jjart, the supracesophageal or cereljral 

 ganglion, and of a ventral part, the so called ventral 

 ganglion chain, connected with one another by a 

 commissure round the æso])hagus (see PI. VIIT, fig. I). 



Tlie cerebral ganglion (see figs. 1. 4 and 17\ 

 wliich is situated in the anterior jtart of the head, 

 just in front of the commencement of the intestine, 

 is of consideralde size, and consists of 2 .symmetri- 

 cal halves connected with one another in the middle. 

 Above, it forms several lobes whicli partially sur- 

 round tlie base of the ocellus (see tig. 4), and, after 

 sending out a fine nerve to each of the first pair of 

 antennæ (a'), continues on each side in the form of 

 the powerfully developed optic nerve (o). A second 

 pair of nerves are seen to issue from the cerebral 

 ganglion farther forward, on each side of the ocellus. 

 They each end in a little ganglionic protviberance 

 immediately behnv a .small pit-like depression in the 

 anterior integument of the Iiead (a) probably an- 

 swering to the ciliated pits occurring in about tlie 

 same place in Limnetic:, and wliich are generally 

 (■onsidered to be a kind of organ of sense. The 

 nerves for the second pair of antennæ (a-) wliicli 

 are considerably stronger in the males tluin in the 

 females, originate from the anterior piut of the 

 U'sophageal commissures. These, before they encircle 

 the (eso])liagus, are connected by a thin transver'se 

 commissure (see figs. 1 and 4) lying near the base 

 (d' tlie anterior lip. 



